The present invention relates to an astigmatism generating device to be used in an optical pickup device or the like in an optical video disk player and a CD player and more particularly to such an astigmatism generating device as is capable of enhancing the efficiency in the utilization of light through a reduction of the loss in the amount of light.
A conventional astigmatism generating device is disclosed in the Official Gazette No. 58316-1988 for Utility Model Laid Open (Application No. 151736-1986) and U.S. Pat. No. 4,805,992, and the device therein proposed is shown in FIG. 6. In the Figure, the existing astigmatism generating device 1 is shown to be composed of a first plane 11, which reflects the divergent light emitted from the light source 2 onto the disk 4 and also receives the incidence of the light reflected from the disk 4, a second plane 12, which is formed in parallel to the first plane 11 and radiates the light transmitted from the first plane 11, and a third plane 13, which is formed at a prescribed inclination angle .alpha. measured from the first plane 11 and reflects the light incident from the first plane 11 mentioned above to the second plane 12, the astigmatism generating device 1 thus having a construction which radiates the light from the second plane 12 mentioned above to the light receiving element 5.
The prescribed angle .alpha. of the third plane 13 mentioned above is set in such a way that the coma aberration of the light radiated from the second plane 12 will be approximately zero, and, specifically, the angle is set on the basis of the relationship expressed in the following equation (1): EQU .alpha.=90.degree.-(.beta.o/2) (1);
Wherein, .beta.o represents the angle of the vertical angle .beta. at which the coma aberration exactly becomes zero in an astigmatism generating device which employs a wedge-shaped flat plate of which the vertical angle of the two planes is expressed as .beta..
The conventional device based on the construction described above can generate only astigmatism without producing any coma aberration since the prescribed angle .alpha. at which the third plane 13 is inclined at is set as described above.
The existing astigmatism generating device is composed as described above, and consequently the device presented a problem in that the efficiency in the utilization of light is deteriorated as the result of the deterioration of the light transmitting efficiency of the second plane 12 since a portion of the light reflected on the third plane 13 is again reflected by the second plane 12. Furthermore, the light reflected by the second plane 12 mentioned above is turned into stray light, causing unfavorable results, such as a deterioration of the accuracy of signal reproduction by the light receiving element 5.
In order to cause an improvement in the light transmitting efficiency of the second plane 12 mentioned above, it has been conventionally suggested to form a device construction which almost completely transmits the light reflected on the second plane 12, by the effect of antireflection coatings formed of magnesium fluoride, etc., with different indices of refraction, applied over the second plane 12. Such a process, however, presents a problem in that the application of such antireflection coatings results in an increased complexity of device construction, and thus requires more elaborate manufacturing processes, and so forth.